1. Field of the Invention
The present invention relates to a process for the safe operation of a continuously operated heterogeneously catalyzed gas-phase partial oxidation of at least one organic compound in an oxidation reactor whose feed gas mixture comprises, in addition to the at least one organic compound to be partially oxidized and molecular oxygen as an oxidizing agent, at least one diluent gas substantially inert under the conditions of a heterogeneously catalyzed gas-phase partial oxidation, in which a cut-out mechanism is used for preventing the oxidation reactor from being fed with a feed gas mixture whose composition is an explosive one.
2. Description of the Related Art
Here, a complete oxidation of an organic compound with molecular oxygen is understood as meaning that the organic compound is converted under the reactive action of molecular oxygen so that all the carbon contained in the organic compound is converted into oxides of carbon and all the hydrogen contained in the organic compound is converted into oxides of hydrogen. All compounds of an organic compound under the reactive action of molecular oxygen which differ from this are summarized here as partial oxidations of an organic compound.
In particular, partial oxidations are to be understood here as meaning those reactions of organic compounds under the reactive action of molecular oxygen in which the organic compound to be partially oxidized contains at least one chemically bonded oxygen atom more after the end of the reaction than before the partial oxidation is carried out.
A diluent gas substantially inert under the conditions of the heterogeneously catalyzed gas-phase partial oxidation is understood as meaning those diluent gases whose components—each considered by itself—remain unchanged to an extent of more than 95, preferably more than 99, mol % under the conditions of the heterogeneously catalyzed gas-phase partial oxidation.
Gaseous mixtures containing molecular oxygen and a partially oxidizable gaseous organic compound are potentially explosive gas mixtures.
What is decisive with regard to answering the question as to whether the gas mixture is explosive or not is whether or not combustion (ignition, explosion) initiated by a local ignition source (e.g. glowing platinum wire) spreads in the gas mixture present under specific starting conditions (pressure, temperature) (cf. DIN 51649). If said combustion spreads, the mixture is to be defined here as being explosive. If no spread occurs, the mixture is classified as nonexplosive in this document.
The investigations required for this purpose are usually carried out in a closed stainless steel 51 high-pressure container. The gas mixture in the initially evacuated high-pressure container is usually produced by the partial pressure method. After mixing for about 10 minutes by means of a magnetic stirrer, an attempt is made to ignite the gas mixture present in each case at a specific starting pressure and a specific starting temperature by means of a melting platinum wire. Any automatic spread of a reaction front (explosion) triggered thereby is detected by the increase in the internal pressure in the container as a function of time (e.g. measurement using a piezoelectric pressure transducer) and by the increase in the temperature in the container (cf. for example EP-A 731080 and DE-A 19622331.
Gases whose mixtures with air are not capable of combustion reaction spreading from an ignition source at any mixing ratios are to be defined here as being nonflammable gases. Typical examples of nonflammable gases are CO2, H2O, N2 and all noble gases.
Gases whose mixtures with air are capable of a combustion reaction spreading from an ignition source at specific mixing ratios are to be defined here as being flammable gases. Examples of flammable gases are hydrogen, ethane, propane, butane and diethyl ether.
While nonflammable gases are inert diluent gases suitable in principle for heterogeneously catalyzed gas-phase partial oxidations of organic compounds, in the case of flammable gases this must be determined by a person skilled in the art with respect to a specific heterogeneously catalyzed gas-phase partial oxidation by means of a few preliminary experiments.
It is generally known that numerous key chemicals can be produced by partial and heterogeneously catalyzed oxidation of various organic compounds with molecular oxygen in the gas phase. Examples are the reaction of propylene to give acrolein and/or acrylic acid (cf. for example DE-A 2351151), the reaction of tert-butanol, isobutene, isobutane, isobutyraldehyde or the methyl ether of tert-butanol to give methacrolein and/or methacrylic acid (cf. for example DE-A 2526238, EP-A 92097, EP-A 58927, DE-A 4132263, DE-A 4132684 and DE-A 4022212), the reaction of acrolein to give acrylic acid, the reaction of methacrolein to give methacrylic acid (cf. for example DE-A 2526238), the reaction of o-xylene or naphthalene to give phthalic anhydride (cf. for example EP-A 522871) and the reaction of butadiene to give maleic anhydride (cf. for example DE-A 2106796 and DE-A 1624921), the reaction of n-butane to give maleic anhydride (cf. for example GB-A 1464198 and GB-A 1291354), the reaction of indanes to give, for example, anthraquinone (cf. for example DE-A 2025430), the reaction of ethylene to give ethylene oxide or of propylene to give propylene oxide (cf. for example DE-B 1254137, DE-A 2159346, EP-A 372972, WO 89/0710, DE-A 4311608 and Beyer, Lehrbuch der organischen Chemie, 17th edition (1973), Hirzel Verlag Stuttgart, page 261), the reaction of propylene and/or acrolein to give acrylonitrile (cf. for example DE-A 2351151), the reaction of isobutene and/or methacrolein to give methacrylonitrile (i.e. the term partial oxidation is intended in this document also to include partial ammoxidation, i.e. partial oxidation in the presence of ammonia), the oxidative dehydrogenation of hydrocarbons (cf. for example DE-A 2351151), the reaction of propane to give acrylonitrile or to give acrolein and/or acrylic acid (cf. for example DE-A 10131297, EP-A 1090684, EP-A 608838, DE-A 10046672, EP-A 529853, WO 01/96270 and DE-A 10028582) etc.
The catalysts to be used are usually solids.
The catalysts used are particularly frequently oxide materials or noble metals (e.g. Ag). Apart from oxygen, the catalytically active oxide material may contain only one other element or more than one other element (multielement oxide materials). Particularly frequently used catalytically active oxide materials are those which comprise more than one metallic, in particular transition metal, element. In this case, the term multimetal oxide materials is used. Usually, multielement oxide materials are not simple physical mixtures of oxides of the elemental constituents but heterogeneous mixtures of complex polycompounds of these elements.
In general, heterogeneously catalyzed gas-phase partial oxidations, in particular the abovementioned ones, are carried out at elevated temperatures (as a rule a few hundred degrees C., usually from 100 to 600° C.).
Since most heterogeneously catalyzed gas-phase partial oxidations are strongly exothermic, they are, for reasons relating to heat removal, expediently often carried out in a fluidized bed or in fixed-bed reactors comprising a multiplicity of catalyst tubes, a heat exchange medium being passed through the space surrounding the catalyst tubes in said reactors.
The operating pressure (absolute pressure) in heterogeneously catalyzed gas-phase partial oxidations may be either below, at or above 1 atm. As a rule, it is from 1 to 10, generally from 1 to 3, atm. The desired reaction takes place during the residence time of the reaction gas mixture in the catalyst bed through which it is passed.
Owing to the generally pronounced exothermic character of most heterogeneously catalyzed gas-phase partial oxidations of organic compounds with molecular oxygen, the reactants are usually diluted with a gas which is substantially inert under the conditions of the gas-phase catalytic partial oxidation and, with its heat capacity, is capable of absorbing liberated heat of reaction.
One of the most frequently used inert diluent gases is molecular nitrogen, which is automatically used whenever air is used as an oxygen source for the heterogeneously catalyzed gas-phase partial oxidation.
Another inert diluent gas often used is steam, owing to its general availability. Moreover, both nitrogen and steam advantageously form nonflammable inert diluent gases.
Often recycle gas is also concomitantly used as inert diluent gas (cf. for example EP-A 1180508). Recycle gas is defined as the residual gas which remains after a one-stage or multistage (in the multistage heterogeneously catalyzed gas-phase partial oxidation of organic compounds, the gas-phase partial oxidation, in contrast to the one-stage heterogeneously catalyzed gas-phase partial oxidation, is carried out not in one reactor but in at least two reactors connected in series, as a rule oxidizing agent being added between successive reactors; multiple stages are used in particular when the partial oxidation is carried out in successive steps; in these cases, it is frequently expedient to adapt both the catalyst and the other reaction conditions in an optimum manner to the respective reaction step and to carry out the reaction step in a separate reactor, in a separate reaction stage; however, multiple stages may also be used when, for reasons relating to heat removal or for other reasons (cf. for example DE-A 19902562), the reaction is spread over a plurality of reactors connected in series; an example of a heterogeneously catalyzed gas-phase partial oxidation frequently carried out in two stages is the partial oxidation of propylene to acrylic acid; the propylene is oxidized to acrolein in the first reaction stage, and the acrolein to acrylic acid in the second reaction stage; in a corresponding manner, the preparation of methacrylic acid is also frequently carried out in two stages, in general starting from isobutene; both abovementioned partial oxidations can, however, also be carried out in one stage (both steps in one reactor) when suitable catalyst loads are used) heterogeneously catalyzed gas-phase partial oxidation of at least one organic compound when the desired product has been isolated more or less selectively (for example by absorption in a suitable solvent) from the product gas mixture. As a rule, it predominantly comprises the inert diluent gases used for the partial oxidation and steam usually formed as a byproduct in the partial oxidation and carbon oxides and steam formed by undesired complete oxidation. In some cases, it also contains small amounts of oxygen not consumed in the partial oxidation (residual oxygen) and/or of unconverted organic starting compounds.
The steam formed as a byproduct ensures in most cases that the partial oxidation takes place without significant volume changes of the reaction gas mixture.
According to the above, in most heterogeneously catalyzed gas-phase partial oxidations of organic compounds the inert diluent gas concomitantly used comprises ≧90, frequently ≧95, % by volume of N2, H2O, and/or CO2 and thus substantially comprises nonflammable inert diluent gases.
However, the inert diluent gases concomitantly used not only ensure, in combination with the other measures used for removing the heat of reaction (e.g. external cooling), a substantially uniform temperature of the reaction gas mixture along the reaction path but also permit the safe operation of a continuous heterogeneously catalyzed gas-phase partial oxidation of organic compounds.
If in fact the explosion behavior of a gas mixture consisting of molecular oxygen, a nonflammable inert diluent gas and a flammable organic compound and present at a specific temperature and a specific pressure is investigated as a function of the composition of the mixture, a result is obtained as shown for a typical case by the explosion diagram of FIG. 1 for the propylene/oxygen-nitrogen gas mixture.
The abscissa shows the proportion of propylene in the mixture in % by volume (VC3), the ordinate shows the proportion of molecular oxygen in the mixture in % by volume (VO2) and the residual amount to 100% by volume is always the molecular nitrogen.
If the composition of the gas mixture is in the hatched area of FIG. 1, it is explosive; if it is outside, it is nonexplosive. The solid line separates the explosive area from the nonexplosive area and is defined as the explosion limit.
If, for example, steam or a mixture of nitrogen and steam were to be used instead of nitrogen as the inert diluent gas, the explosion limit would be virtually coincident. This also applies to most other nonflammable inert diluent gases, as long as their specific molar heat does not differ considerably from that of molecular nitrogen. The pressure and temperature dependence of an explosion diagram is limited and, where changes are not too large, can be neglected.
If the propylene were to be replaced by a mixture of organic compounds of a specific composition or by another organic compound, the qualitative curve of the explosion limit in the explosion diagram would be qualitatively the same.